This invention relates to a caliper brake for a bicycle, whose brake shoes are pressed against side faces of a rim of one wheel of the bicycle to brake it.
In general, there are three kinds of caliper brakes, center-pull, side-pull and cantilever types. The brake of the invention belongs to the cantilever type caliper brake.
Before explaining conventional brakes, frame members of a bicycle are explained referring to FIGS. 2a-2f and 8a and 8b. A bicycle as shown comprises a head tube 1, fork blades 2, a front wheel 3 including a tire 3a and a rim 3b, a top tube 4 (FIG. 2a), a front or down tube 5, a seat tube 6, chain stays 7 and seat stays 8. Another bicycle shown in FIG. 2b comprises a main frame 9. The bicycle further comprises a band brake 10, a rear wheel 11 having a wheel 11a and a rim 11b, a bottom bracket 12, chain wheel cranks 13 and a chain wheel 14.
FIG. 1 illustrates one example of a brake of the prior art for a rear wheel. As shown in the drawing, support bases 15 are fixed to the seat stays 8 for pivotally supporting brake members 16 in the form of cantilevers with the aid of bolts 17. The brake comprises brake arms 16a, brake shoes 18 secured to the brake members 16, a connecting wire 19 for connecting the brake arms 16a, and an operating wire 20 connected to a center portion of the connecting wire 19 for operating the brake. FIGS. 2a and 2b illustrate a diamond-shaped frame and a U-shaped frame for bicycles, respectively, and FIGS. 2c and 2d illustrate staggered frames and FIGS. 2e and 2f illustrate roop-shaped frames.
In case of the diamond-shaped frame of the bicycle shown in FIG. 2a, an operating wire 20 connected to brake levers (not shown) provided at handlebars can be easily extended rearwardly along the front tube 5 as shown in a solid line or along the top tube 4 as shown in two-dot-and-dash line without any trouble.
With the bicycles shown in FIGS. 2b-2f other than the diamond-shaped frame bicycle, however, there are many sharply bent portions of the operating wires 20 in order to extend the wire 20 from the handlebars to the seat stays 8. As a result, frictional resistance in the brake wire consisting of an inner and an outer wire disadvantageously increases.
In order to avoid such many bent portions of the brake wire, the brake may be provided on the chain stays 7. FIG. 3 illustrates the brake of the prior art mounted on the chain stays 7. With this arrangement, however, the arms 16a of the brake members 16 would interfere with the chain wheel cranks 13. Therefore, it is impossible to mount the brake on the chain stays 7.
FIG. 4 illustrates a center pull type caliper brake of the prior art mounted on the chain stays 7. Referring to FIG. 4, curved caliper brake arms 21 are pivotally connected with their centers to the chain stays 7 by means of bolts 22 and have brake shoes 23 fixed to their ends by means of bolts 24 in opposition to rims 11b of a wheel of a bicycle. Ends 21a of arms of the caliper brake arms 21 intersecting with each other are interconnected by a connecting wire 25 whose center is connected to an operating wire 20 by a connecting metal member 26.
In this case, however, the ends 21a of the caliper brake arms 21 interfere with the chain wheel 14 like in the case discussed above. Therefore, such a mounting of the brake on the chain stays 7 is also impossible.
In the event that the ends 16a or 21a of the arms interfere with other components of the bicycle, such an interference of the arms may be avoided by limiting the outward extensions of the arms. However, as the rotating radii of the arms become smaller, a force acting upon the operating wire must be increased in order to obtain a pressing force substantially equal to that of the case having the previous longer rotating radii.
However, operating wires are generally used at uppermost limits allowed by their mechanical properties. If the operating wires are subjected to larger forces in use, they would exhibit excessive elongations and the larger forces detrimentally affect their strength and durability.
It has been proposed to use a running block unit having two pulleys rotatably connected by two side plates for a center-pull type brake of a bicycle as disclosed in Japanese Utility Model Application Publication No. 52-3,799. In more detail, an operating wire having one end fixed to a frame of the bicycle extends about one of the pulleys and the other end is pulled when a brake action is applied to the brake. On the other hand, ends of caliper brake arms are connected by a connecting wire whose center extends about the other pulleys.
With this arrangement, since the running block unit is moved a distance one half of a moved distance of the operating wire pulled by an operation of brake levers at handlebars, the force caused by the connecting wire connecting the caliper brake arms is advantageously twice the force on the operating wire caused by the brake levers. However, since strokes of brake shoes become one half of original strokes in case directly pulling the connecting wire without using the running block unit, clearances between the brake shoes and a rim of a wheel of the bicycle must be one half of original clearances. Therefore, the brake shoes are likely to contact the rim of the wheel and require frequent adjustment of clearances particularly in the event that the connecting wire connecting the caliper brake arms have been elongated and movable portions have been worn.
In the case of using the running pulley block in order to reduce the brake operating force to be applied to brake levers at handlebars by a cyclist, strokes of the brake shoes are unavoidably reduced. To eliminate such a disadvantage, strokes of the brake levers must be twice those in the case without using the running pulley block. However, such long strokes of the brake levers would make operation of the brake difficult. According to the invention the operating radii are made small so that even if the strokes of the brake levers remain unaltered, sufficient strokes of the brake shoes and sufficient clearances thereat are obtained.
Moreover, outwardly extending portions of the brake interfere with the feet and clothes of a cyclist and the brake itself is likely to be damaged when the bicycle falls down.